We are developing, in healthy subjects, a method to measure synaptic dopamine levels by measuring the number of dopamine D2 receptors unmasked when dopamine is depleted with a synthesis inhibitor (alpha-methyl-para-tyrosine). Using a new SPECT (single photon emission computed tomography) radiotracers, we have been able to measure both striatal and cortical dopmainergic transmission. Preliminary analysis has shown that approximately 20-40% of D2 receptors in cortex and striatum are occupied by dopamine.